The parasitemia of Trypanosoma lewisi is characterized by a host mediated inhibition of parasite reproduction and subsequent termination of the infection. This research is designed to: (1) characterize morphologic, reproductive, metabolic and antigenic changes in the parasites during the infections, (2) determine the mechanisms of host regulation of the parasites, and (3) elucidate the processes which mediate functional immunity to T. lewisi and other lewisi group trypanosomes. The factor, ablastin, responsible for the inhibition of reproduction is being isolated and characterized by gel filtration chromatography, immunospecific adsorption and acrylamide gel electrophoresis. The procedures are coupled with assays of specific activity on bloodstream forms grown in vitro at 37 degrees Centrigrade. Transport and membrane studies are conducted with isotopically labeled amino acids and nucleosides in the presence of normal and ablastic sera. Charge and conformational alterations are examined using DEAE chromatography, measurements of sodium and potassium fluxes and membrane potentials and electron paramagnetic resonance spectra of spin- labeled lipophilic compounds. The electron microscope will be used to localize ferritin-labeled serum proteins on the membranes of the trypanosomes. Serologic analyses and protection studies with lewisi group trypanosomes include cell fractionation and the isolation and characterization by chromatography and electrophoresis of antigens involved in provoking a protective immunity to homologous and heterologous trypanosomes.